Method of blowing aluminum



Sept. 19, 1967 J o s ET AL METHOD or" BLOWING ALUMINUM 4 Sheets-Sheet 5Filed May 14, 1964 FIG. 8

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FIG. 9

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INVENTORS kfiikg a T' BY W/W VW ATTORNEYS Sept. 19, BQN|$ ET AL METHODOF BLOWING ALUMINUM Filed May 14, 1964 4 Sheets-Sheet 4 FIG. IO

INVENTORS LASZLO J. BONIS AUGUST F. WITT n w WW ATTORNEYS United StatesPatent 3,342,248 METHOD OF BLOWING ALUMINUM Laszlo J. Bonis, Brookliue,and August F. Witt, Arlington, Mass., assignors to Ilikon Corporation,Natick, Mass, a corporation of Delaware Filed May 14, 1964, Ser. No.381,278 33 Claims. (Cl. 164-55) This invention relates to an improvedmethod of fabricating articles and is particularly directed to a novelmethod of blowing hollow articles directly from a fused or liquidmaterial. This application is a continuation-inpart of our copendingapplication, Ser. No. 274,381, filed on Apr. 15, 1963, now abandoned,which was a continuation-in-part of our application, Ser. No. 188,473,filed Apr. 18, 1962, now abandoned.

An object of our invention is to provide a novel method of blowinghollow articles, such as containers and the like, directly from areservoir of material maintained in a liquid state.

Another object of this invention is to provide a method of blowinghollow, thin-walled articles directly into an open-ended die from areservoir of molten material.

Another object of this invention is to provide a method of blowing ahollow article with a liner.

A further object of this invention is to provide a rapid, economicalmethod of producing an article of manufacture directly from a liquid orsemi-liquid material.

Yet another object of the invention is to provide a method of shaping afilm of liquid or semiliquid material in a die and stabilizing thematerial in the die to produce a solid article of manufacture.

A more specific object of the invention is to provide a method ofblowing aluminum directly into an open-ended die from a reservoir ofmolten aluminum to form hollow, thin-walled articles. l

Other objects of our invention will be apparent to those skilled in theart from the following description.

The objects of this invention are achieved by a method whereby a moltenmaterial or other material capable of being blown while in a fused orliquid state, is contained in a supply reservoir and an article is blowndirectly from the reservoir into a die suspended over the reservoir, byreleasing a blowing fi-uid below the surface of the liquid material. Theterm blowing as used herein is used to indicate the action of theblOWing fluid, such as an expandable gas under pressure,.in forming abubble beneath the surface of the liquid material in the reservoir andrising to carry into the open end of the die, and shape therein, a thincoherent shell of thematerial from the reservoir. The term blowing asused herein is not used in the sense of blowing to provide a foam or amulticellular structure.

In accordance with one aspect of this invention, a quantity of material,such as metal, is melted in a supply reservoir or vessel in which isdisposed at blowpipe having an orifice directed upwardly toward andspaced -a predetermined distance below the surface of the molten metal.In this regard it should be noted that the term metal is used herein inits usual metallurgical sense, and not in the sense at one time commonin the glassmaking art to mean molten glass. A hollow die, characterizedby a die cavity internally formed to shape the desired article, issuspended over the vessel in registry with the orifice of the blowpipe.

A predetermined minimum volume or charge of pressurized gas isintroduced into the blowpipe to form a bubble at the orifice. Thepressure of the gas should be sufiicient to overcome the hydrostatichead of liquid on the reservoir above the blowpipe. Where a die isemployed the gas pressure also should be sufficient to permit the bubbleto enter the die cavity i.e. suflicient to overcome the overpressurewithin the die, and to expand upwardly into the desired contact with theinternal walls of the die before solidification. The bubble expands andrises to carry into intimate conformity with the die a thin, coherent,film-like shell of metal directly from the molten supply. When thebubble has reached its desired limiting or constraining position in thedie, it is stabilized into the desired form by solidification, chemicalreaction, polymerzation, heating, cooling or the like. After the bubblehas blown the article into the die, the latter is removed from itsposition above the surface of the melt'to permit extraction of thesolidified article.

Referring to the drawings: FIGURE '1 is a diagrammatic representation ofan apparatus used in practicing the method of our invention.

FIGURES 2 through 5 are diagrammatic representations of the use of ournovel blowing method, in fabrieating a hollow sealed article.

' FIGURES 6 through 8 are diagrammatic representations of our novelmethod used in fabricating a hollow article with a liner.

FIGURE 9 is a cross-sectional view of the article formed by the methodof FIGURES 6 through 8.

FIGURE 10 is a diagrammatic view of a modified form of the apparatusshown in FIGURE 1..

It is believed that the nature and types of articles which are capableof being fabricated by our method will be understood from a descriptionof the method as practiced in forming hollow container bodies fromaluminum alloy.

With reference to FIGURE 1, a quantity 10 of a suitable aluminum alloyis contained in a vessel 12 and mainmelt 10 with its open end facing andin registry with is'stretched and pressed against the walls the blowpipeorifice 18. As shown, the open end of the die may be immersed slightlybelow the melt surface for reasons to be explained hereinafter. The dieis internally configurated with a cavity shaped to form a tubularcontainer, and is closed at its upper end except for a vent hole-22through which the air within the die cavity may be evacuated during theblowing of the container.

As best illustrated in FIGURE 2, a predetermined volume of gas releasedfrom its source through blowpipe 16 causes a bubble 24 of moltenaluminum to rise from orifice 16 and expand upwardly through the melt10, above the melt surface and into the cavity of the die 20. The bubbleexpands so that the film continually expands and of the die cavity tosolidify and form a thin, coherent, stabilized shell 26 in the shape ofthe desired container, as seen in FIGURE 3. The die 20 is then removed,the gas within the die permitted to escape, and the solidified,thin-film article as used herein includes a thin film of liquid materialinflated with a fluid such as gas.

A specific example of the practice of our method involved the use of acylindrical cast iron die having a cavity shaped to form a containerhaving a diameter of 1.5" and a length of 2.5. The material used wasASTM aluminum alloy 2024 and was maintained in a molten state at atemperature of about 1200 F.

The blowpipe orifice had a diameter of 11 millimeters, and the distanceX of the orifice below the melt surface was approximately 15millimeters. The blowing gas was a mixture of about 97 parts nitrogenand about 3 parts oxygen, by volume, and was released through theblowpipe in charges of predetermined volume corresponding to a sourcepressure of approximately 25 mm. Hg above atmospheric pressure and atime interval of about one second.

The open end of the die was immersed below the melt surface a distance Y(FIGURE 2) of about millimeters. At these conditions, hollow containershaving the above length and diameter dimensions were blown successivelywith fairly uniform wall thickness of about .050".

The above-described method may be used to blow hollow articles from anyliquid material which is capable of forming a continuous expandablebubble by the action of a blowing fluid released below the surface ofthe liquid material. The term liquid as used herein includes molten,fused, semiliquid, liquid dispersion, liquids or any fluidtype materialcapable of being blown into a bubble, and which materials are capable ofbeing stabilized such as by solidification, fusion, polymerization, orthe like. Suitable materials would include metals, metal alloys, andnatural and synthetic organic materials such as plastics, resins, andmonomeric and polymeric materials either in bulk, dispersion, orsolution form. These materials may be blown from the bulk material inmolten or syrup form, from an aqueous or organic solvent solution of thematerial, from latices, and from dispersions of the material indiluents, plasticizers, 'or other liquids. These materials may bestabilized after being blown or partially stabilized prior to or duringthe process of being blown. For example, stabilization may be affectedby maintaining the die or other article forming means or the blowing gasor the material at a high or low controlled temperature or bypolymerizing the material through employing a blowing fluid containingan accelerator, a curing agent or a polymerization catalyst or otherwisecontacting the material being blown, the blown material or the internalwalls of the die cavity with additives or catalysts suflicient to affectthe desired stabilizing condition either alone or in combination withthe employment of controlled temperatures of the die and blowing fluid.

Suitable specific materials would include, but not be limited tothermoplastic and thermosetting resins such as vinyl resins likepolyvinyl chloride and copolymers of vinyl chloride and other vinylhalides with vinyl esters of short-chain fatty acids like vinyl acetate,and other ethylenically unsaturated modifying monomers or polymers.Hollow plastic articles may be blown from solvent hydrocarbon solutionsof the resins or from dispersion of the resin particles in suitableplasticizers to form vinyl plastisols or solvent-plasticizercombinations to form vinyl organosols. For example, hollow vinylarticles may be formed by blowing a vinyl plastisol bubble into a diewhich is maintained at a temperature above the fusion temperature of thevinyl resin whereby stabilization is effected by the fusion and coolingof the blown vinyl film within the die.

Other materials include thermosetting resins such as phenol-aldehyde andresorcinol-aldehyde and other such resins prepared either under acid,alkaline or neutral conditions, either as two-step novolak or asone-step resol resins. For example, a resol-type, phenol-formaldehydeaqueous resin solution containing a smallamount of hexamethylenetetra-amine may be blown as a bubble into a die maintained at atemperature sufficiently high to induce the breakdown of thehexamethylene tetra-amine whereby sufficient methylene radicals aregenerated to effect stabilization of the blown phenol-formaldehyde resinby cross-linking to form the infusible, insoluble resin product. Atwo-step resin, that is, a resin containing less than a stoichimetricamount of formaldehyde sufficient to produce the insoluble resinousproduct, may also be blown from a phenol-formaldehyde solution byemploying a blowing gas containing formaldehyde whereupon theformaldehyde in the blowing gas combined with the temperature of the dieinto which the phenol-formaldehyde resin is blown, is sufficient toeffect stabilization of the resin by a cross-linking chemical reaction.

Other organic material prepared by polymerization or cross-linkingreactions may be blown such as the polyether and polyester-type urethaneprepolymers and polymers, as well as regular polyester resins,polyamides (nylon), epoxy resins, and the like. For example, theseresins may be blown from a solution and stabilized by employing apolymerization catalyst or cross-linking agent in the blowing fluid orwithin the die prior to blowing the material. If desired, the materialor one or more reactants may be blown into the die and stabilizedsubsequently by the introduction into the die of a necessary catalyst,stabilization additive or other reactant to complete the reaction toform the stabilized hollow plastic article.

Other suitable material includes natural and synthetic elastomers, suchas natural rubber and the conjugated diene polymerizates such asstyrene-butadiene copolymers, acrylonitrile-butadiene-styrenecopolymers, butadieneacrylonitrile rubbers, polybutadiene, polyisoprene,butyl rubber, halogenated butyl rubber, ethylene-propoylene copolymersand other elastomeric-type materials including elastomeric urethanes.These materials may be stabilized by being vulcanized or cured in aconventional manner such as by employing curing agents and acceleratorsand a heated die. Curing may be effected partially in the reservoir orthe curing agents and accelerators may be incorporated into the blowingfluid or onto the internal walls into which the material is blown andthe die maintained at a temperature sufiicient to induce rapid curing orvulcanization. These materials may be employed in a latex or hydrocarbonsolution form.

Thermoplastic resins may also be employed with our methods, thesematerials being characterized by being plastic or by assuming a moltenform upon being heated.

Suitable specific materials may include polyolefin resins such aspolyethylene and polypropylene, vinyl resins such as polyvinyl chlorideand polyvinyl chloride-vinyl acetate copolymers, styrene polymers andcopolymers such as polystyrene, styrene-butadiene copolymers, acryliccopolymers such as homo and copolymers containing a hydrocarbon ester oraminoester of an acrylic acid or a 'methaacrylic acid, protein resins,alkyd resins, cellulosic resins as well as other natural resins such aspetroleum and polyterpene resins, and the like. These materials may beplaced in condition for blowing by dissolving in a suitable solvent orblown in bulk from a molten solution provided the temperature is notsufliciently high to cause charring or degradation of the effectiveproperties of the thermoplastic material. Additives such as plasticizersmay be incorporated into the materials prior to being blown. Thestabilization of these materials after being blown may be effected byblowing the material from a bulk solution into a chilled die whereuponthe hot film thermoplastic resin is sufiiciently cooled upon contactwith the internal walls of the die to effect stabilization into thedesired hollow plastic article. Where the material is blown from asolvent solution, stabilization may be effected by the rapid removal ofthe solvent such as by using a vacuum or high and low temperatures ofthe die.

As can be seen, a wide variety of materials may be successfully employedin our novel process provided the material is capable of being blowninto a bubble and stabilized. Additives, such as curing agents,accelerators, fillers, carbon black, plasticizers, stabilizers,antioxidants, lubricants, greases, oils, solvents, colorants, dies,fibers, chemical blowing agents, metal salts, catalysts, clays, metalpowder and the like, may be incorporated into the materials to be blown,introduced with the blowing fluids, or introduced into the die eitherbefore or after forming the hollow article. Additionally, where desiredhollow articles containing a combination of properties may be obtainedby the formation of multilayers of different materials within the samedie by a series of sequential blowing steps employing the same die whereventing means are provided after the formation of each blown layer.Particularly advantageous may be the method of employing a reservoircontaining a monomer or monomer-polymer syrup with an additive amount ofa polymerization catalyst and blowing the material into a die which dieis maintained at a temperature suflicient to induce polymerization.Based on the foregoing discussion the die or other means to form theexpanding bubble into the desired articles should preferably be capableof having a controlled temperature.

Metals and metal alloys appear especially suited for blowing by thismethod because of their ability to quickly stabilize and form a thincoherent bubble film. Aluminum alloys in which additive elements such asGroup VI elements like sulphur, oxygen, selenium, or other elements orcombinations thereof are present in the base metal, may be blown.Usually, these additives are present in the alloy ingot or are added tothe base metal in the melt; however, additives may be incorporated intothe blowing fluid and thus added to the base metal during the blowingstep. For example, a sulphur-containing gas such as sulphur dioxide orhydrogen sulfide may be used as the blowing fluid to obtain desiredproperties.

Aluminum alloys appear especially convenient for use in this methodbecause of their high strength-to-weight ratio, reasonable meltingpoint, and their ability to stabilize rapidly due to a desirable rangeof constitutional supercooling (i.e. solid plus liquid phase region).Aluminum alloys are noted for their ability to oxidize almostinstantaneously upon contact with an oxidizing gas such as oxygen. It isbelieved this contributes to the ability to rapidly stabilize in theform of a continuous, relatively thin, strong, coherent, filmlike layerin that as the bubble forms, a thin layer of aluminum oxide is createdon the surface of the bubble exposed to the oxygen. This oxide filmforms on the interior of the bubble due to the presence of oxygen in theblowing gas. A like oxide film may form on the exterior of the bubble asit rises above the surface of the melt in those instances whereoygen-containing atmosphere, such as air, is present in and around thedie. The term film-like particularly characterizes the surface filmformed, which may be a mono or polymolecular layer, such as amonomolecular layer of an oxide of aluminum.

Among the aluminum alloys which have produced satisfactory results areASTM 3003, 2024, 2025, 4043, B195, B750, A356, C612F andaluminum-silicon binary alloys are preferred where it is desired toobtain a ductile, grainoriented product having a relatively smoothfinish, such as a can for packaging comestibles. ASTM 2024 being arelatively low-grade commercial alloy, its use will result in savings inthe cost of manufacture as compared to the use of specially compoundedalloys.

Other metals which have oxide-forming and rapid stabilizationcharacteristics similar to that of aluminum alloy may be used with thismethod. Many metal alloys are capable at their relatively high meltingtemperatures and above of rapidly forming filmlike surface layers suchas of oxide, sulfides, selenides and combinations thereof and the like.For example, the metal alloys and metals which appear particularlysuitable include those which are oxidizable such as aluminum alloys andwhichare subject to almost instantaneous oxidation or reaction in amolten or semi-molten condition with the atmosphere or blowing gas. Insome cases it is desirable to blow the bubble into a nonoxidizingatmosphere or an inert atmosphere, even though the metals are capable ofbeing oxidized. For example, metals which oxidize in an undesirablemanner, such as copper which may form an unsightly green oxide, ormagnesium which in many forms is too reactive to be blown into an oxygenatmosphere or metals which form very brittle oxides, such as stainlesssteel and the like, may be blown into an inert or nonoxidizing gaseousatmosphere with an inert gas such as argon, nitrogen and the like, or atleast a gas containing a considerably reduced amount of oxygenconcentration in the gas.

A wide variety of metals and metal alloys can be blown by this process.Those metals or metal alloys like aluminum and aluminum aloy on beingblown, form a discernible oxide skin; however, other non or lessoxidizable metals may also "be successfully blown into hollow articles.Some metals which by their inherent nature will not form thin film-likeoxide layers may also be successfully blown with this process. Forexample, those metals which ordin-arily have the ability to stabilize.rapidly, but in which an oxide film is undesirable or which does notform filmlike oxide layers on being blown, may be used. These metalsinclude, but are not limited to: lead, tin, cadmium, nickel cobalt,zinc, iron, silver, gold, bismuth, platinum, palladium and the like, aswell as their alloys such as brass, bronze, Woods metal and the like.Metals which are embrittled by oxidation or the reactions such as castiron may be successfully blown in an inert or non oxidizing atmosphere.Some metals inherently brittle may require a controlled rate ofsolidification in the die. The metal bubble upon expanding andcontacting the internal wall of the die forms a cooled and solidifiedfilm-like layer.

A number of variables in the method above described will have an effecton the final article porduced. It is apparent that one of these is thetemperature of the melt. When the melt is maintained at a temperaturejust above the solidus point, the viscosity of the metal will begreaterthan when a higher temperature is utilized. Thus, when the bubbleis formed in the higher viscosity material,

there will be a tendency for a greater amount of material to form aroundthe bubble, thereby resulting in a greater amount of material beingblown into the die. Conversely, a higher temperature with attendantlower viscosity will result in less material being blown into the die bythe bubble. Thus, it will be seen that melt temperature is one factorthat can be controlled tovary the wall thickness of the resultingarticle.

The temperature of the'die or any portion thereof may be varied asdesired to aid in controlling the thickness of the hollow article and inpromoting stabilization of the bubble. In particular, the die may bemaintained at a lower temperature than the temperature of the melt toeffect a rapid stabilization by solidification and cooling of the moltenmetal or plastic material. Conversely, the die or portions thereof maybe maintainedat a higher temheated to produce the desired uniformity ornonuniformity in the hollow article so-formed or all or a portion 'ofthedie may be fabricated of nonheat conductive or less heat conductivematerial than other portions. Likewise, thematerial of which the die isconstructed and internal surface of the die may be so selected topromote adesired surface finish or heat transfer characteristics. Forexample, where a simple die is used as shown in FIGURES 1 through 9,'thecontact of the die with the melt would normally cause a heating of thedie to some equilibrium temperature. However, thetemperature at the topof the die generally will be less than that at the bottom of the diewherein it contacts the melt. Where desired, this difference innonuniformity of die temperature may be further increased in order toobtain progressive stabilization from the one part to the other part ofthe die after the blown material has contacted the die surface. Ifdesired, the die may be maintained at a uniform temperature in order tocontrol the wall thickness of the hollow article. Progressivestabilization sometimes is desirable so that the die may be readilyremoved from the melt while the lower material is still in a rathermolten condition.

Another factor that will have an effect on the final blown article isthe distance X between the blowpipe orifice and the surface of the melt.It has been found that the orifice must be maintained at a certainminimum distance below the surface of the melt in order for properformation of the bubble to occur. If this distance is less than theminimum, the bubble will not fully form and an inadequate amount ofmaterial will be carried into the die, thus resulting in an incompletelyformed article, such as the formation of a thin-film article containingholes in the film. When the depth of the blowpipe orifice below thesurface of the melt is too great, the diameter of the bubble expandingas it moves upwardly through the melt, at the surface of the melt willbe greater than the diameter of the cavity of the die and will thus notenter the die cavity or not enter properly, resulting in a poorly formedblown article. As the depth of the blowpipe orifice is increased thereis the additional possibility of multiple bubble formation. The blowpipeorifice depth also influences the film thickness of the top portion ofthe article formed and the total amount of material carried into themold. If the blowpipe orifice is too low the expanding bubble of gascarries increasing quantities of material on top of the bubble audit theorifice goes even lower, then mutiple bubble formation results.

The volume of the gas introduced into the blowpipe is varied dependingupon the size of the article to be blown. In other words, the larger thearticle, the greater the volume of gas.

, Further, when blowing large objects or ones having an irregular ornonsymmetrical shape, it may be found advantageous to induce a partialvacuum, for example, on the order of to 20 mm. of mercury, to a sectionor sections of the die into which it may be difiicult to blow the metalbecause of the final size or shape of the blown article. This partialvacuum, which may be induced through the air vent 22, will tend to drawthe bubble into the die and assist in the full formation of the desiredblown article. The creation of a vacuum within the die may be properlytimed with the introduction of the bubble into the die. Creation of acontinuous vacuum may tend to raise the level of the material into theopen end of the die. Elongated hollow articles such as tubes and tubularcontainers may be fabricated by a die which contains a movable fittedpiston or cap member. The piston is moved upwardly or away from thebubble as the bubble rises within the die thereby drawing the bubbleinto the desired form.

The ability and rate of expansion of the bubble into and within the diecavity depends in part upon the overpressure of the atmosphere withinthe die cavity. Where there are means for rapidly expelling theatmospheric pressure within the die cavity as the bubble expandsupwardly therein such as by the employment of large vents, little if anyoverpressure develops and the bubble easily enters, rises and forms thedesired article in the die. Where small vents are provided the rate atwhich the rising molten bubble is permitted to rise and expand withinthe die is decreased and the bubble may solidify prematurely within thedie. If the overpressure is too high the bubble will have difliculty inentering the open end of the die. This results in an undesired andimproperly formed hollow article. It is, therefore, desirable that means8 be provided for a fairly rapid expelling of the gaseous atmospherefrom the die cavity and at least a suflicient rate to prevent prematurestabilization of the bubble.

One method of producing hollow objects is to provide a double open-endeddie with one end of the die immersed in the melt and the other end ofthe die open so that as the bubble expands in the die cavity, nooverpressure or little overpressure is created therein. The top lid ofthe die is then put on just before the bubble reaches the open end ofthe die, so that the article may be formed. This method restrains thebubble to the desired diameter and reduces problems associated withoverpressure within the die cavity. The bubble material should be formedby the introduction of a gas at a minimum gas volume at a predeterminedpressure and a predetermined time flow rate sufficient to have thebubble overcome the hydrostatic head and the die overpressure wherebythe bubble will expand and fill the entire die cavity.

Numerous other factors, such as the diameter of the blowpipe orifice,bubble volume, blowing fluid, and melt material are variables effectingthe ultimate article to be formed.

Various articles have been blown from ASTM 2024 aluminum with a varietyof blowing gases. Such mono and polyatomic gases as nitrogen, argon,helium, oxygen, air, and mixtures of these gases have been used. When itis desired that an oxide film be formed during the blowing of aluminum,the quantity of oxygen necessary in the blowing gas is relatively small.It should be noted, however, that the preferred oxygen content be fromapproximately 1 to 3 parts to parts of the remainder of the gas. It willbe understood that our method is not limited to true gases where theoxygen is in the form of 0 but also includes the use of fluids such asgaseous carbon dioxide, which, upon heating in contact with moltenaluminum, will liberate sufficient O to create the oxide film.

Many blowing fluids can be employed in this invention. These includethose relatively inert fluids which may be used with those blowingmaterials where the article to be 'blown does not have an oxide film, orother gases which react with the material to form desirable materialsand impart desired properties to the article. Other fluids include thosewhich react with the blowing material to form a filmlike oxide layer, asis the case with aluminum and oxygen as above described.

In the practice of this invention, the molten materials are essentiallystabilized by the transformation of a liquid phase to a solid phase.However, where desired, stability may be fully obtained or at leastaided by chemical reaction of the blowing fluid with the molten materialas previously described. The selection of the particular fluid dependsupon the material to be blown and the article desired, whether thestabilization be accomplished by phase transformation or by chemicalreaction.

Suitable gaseous fluids that may be employed in this method includewater vapor, helium, hydrogen, carbon dioxide, carbon monoxide,hydrocarbon gases such as methane, ethane, butane, nitrogen andnitrogen-containing gases such as oxides of nitrogen, sulphur-containinggases such as sulphur dioxide and hydrogen sulfide, halohydrocarbonssuch as fluoro and chloro methane and ethane, halogens, rare earthhalides, argon and other gases.

Suitable liquids that may be utilized as the blowing fluid includesilicone fluids and polysilicones which remain stable at temperatures ofthe molten material, or liquids which decompose to form blowing gases orreactive elements.

Fluidized solids also may be used as the blowing medium. These includegases containing finely divided 7 metal, metal oxide, or other additiveor alloying elements in the fluid stream.

In order to insure the proper formation of the blown article, it isdesirable that the interior surfaces of the die be dry. If, for example,water should be present on the inside surfaces of the die as thealuminum bubble is blown, the hot material will vaporize the water, thusdeforming the bubble and producing an uneven surface on the article.

In most operations an air atmosphere above the melt will permit.satisfactory operation of the blowing process. However, where the moltenmetal is one that is readily oxidized by air, a dross will form on thesurface of the melt and tend to interfere with the efiicient operationof the blowing process in that the dross could be carried into the diewith the bubble. To obviate this problem, a controlled atmosphere ofnitrogen or other non-oxidizing gas may be maintained about the bathand/ or within the die. Alternately, a mechanical skimming operation maybe employed to remove the objectionable dross from the melt surfaceprior to each blowing cycle.

It is also within the scope of our invention that a thin layer of lessdense, nonsoluble, nonoxidizable material, such as molten glass beemployed above the molten metal alloy surface in the reservoir toprevent or reduce oxidation of the lower metal being blown. In thisembodiment a thin layer of A" to /2" of molten glass may be usedwhereupon blowing the bubble of metal alloy, a bubble containing a thinouter layer of glass and a thin inner layer of aluminum is formed andcarried into the die cavity to form a metal-lined glass article. Theupper layer of the reservoir may be any inorganic or organic materialeither capable of being blown or which serves as a protective layer andparts when a bubble is blown.

From the foregoing description it will be apparent that this inventionwill provide a relatively thin-walled, hollow article in a manner whichachieves economy and simplicity of operation. Additionally, the articleoffers distinct advantages over articles of similar types fabricated byconven tional casting and cold forming operations. This method may beemployed to form numerous articles having any desired regular orirregular shape, such as containers, hubcaps, light sockets, trays, etc.

In another form of the invention, specifically illustrated in FIGURES 2through 5, sealed hollow articles may be produced. More particularly,the method may be used to provide, for example, a cylindrical hollowarticle closed at both ends. In this case the bubble 24 is blown inthemanner as describedabove (FIGURES 2 and 3). After the thinshell 26 isblown into intimate contact with the die cavity, an imperforate plate 28is placed under the open end of the die 20. The plate should cover theentire open end of the die. The die and plate are then removed from themelt, whereby the plate traps a layer 30 in the lower open end of thedie. The thickness of this lower layer substantially corresponds to thedistance the die is immersed in the melt, when the plate closes the die.It will be understood that when desired the die may be lifted partiallyout of the melt to an immersed depth Z (FIG- URE 3) before the plate isplaced under the open end of the die, in order to provide a thinnerlayer 30. Upon cooling, this layer forms an integral end wall for thearticle. The blowpipe 16 is preferably moved out of a blowing positionan amount sufficient to enable plate 28 against the open end of the die20.

If a thicker end wall 30 is desired, the distance Z may be variedacocrdingly. The blowing fluid, used to blow the article 26 is entrappedwithin the article. It is readily apparent that the melt surface belowthe trapped bubble within the die 20 will be slightly lower than thesurrounding melt surface as shown in FIGURE 3. This factor must beconsidered in selecting the immersion depth Z to accomplish the desiredthickness of the end wall 30. If desired, the entrapped blowing fluidmay be exhausted from the die 20 prior to placing plate 28 beneath thedie, by first raising the die out of the melt and then lowering it againto the predetermined distance Z.

After the die 20 and plate 28 are removed from the melt 10, the end walllayer 30 solidifies integrally with the shell 26 to form a sealedhollowarticle (FIGURE 5).

placement of the In still anotherform of the invention, lined articlesmay be blown. FIGURES 6 through .9 illustrate this aspect in theformation of a glass-lined metal container. A quantity of molten glass34 is first placed on the orifice 18 of the blowpipe 16 by suitablemeans. A predetermined volume of blowing fluid released through theblowpipe 16 causes a glass bubble 38 to rise from the orifice 18 andexpand upwardly. As the glass bubble 38 rises, a thin layer 40 ofmaterial from the melt 10 will adhere to the exterior surface of thebubble and be carried with the expanding bubble into intimate conformitywith the die cavity walls.

FIGURE 8 shows the formation of a blown glass-lined container 42 whichconforms to the inner shape of the die 20. As shown, the die has beenraised up out of the melt, resulting in the glass being pulled into avery thin bubble 44 and outwardly expanded by the pressure of theblowing fluid. On solidification of the enclosed article 42, the thinwalled glass bubble 44 is broken, and the edges of the article 42 at theopen end of the die 20 smoothed by a torch or other conventionalfinishing means. The article is then removed from the die providing thefinished glass lined container 42 shown in FIGURE 9. It is contemplatedthat such container 42 or other preformed hollow articles may besubsequently lined with an inner layer of another material by employingthe container as a die, with orwithout a supporting die fixture, for asubsequent blowing operation.

As described, the lining material blown may be glass, having a softeningpoint below the temperature. of the melt or otherwise capable of beingplaced in a sufiicient softened state to be blown. Any material such asplastics, metal alloys, vitreous amorphous materials and the like,besides glass, which are compatible and capable of withstanding thetemperature of the melt material, may be used as the inner liner ofarticles prepared in the foregoing manner. The wall thickness of theliner material and, to some extent that of the outer shell, may becontrolled by the quantity of material used, its nature and the blowingpressures employed. The glass-metal layers formed by this method have avery high bond strength.

As indicated in FIGURE 1, the open end of the die 20 is immersedslightly below the surface of the melt. This is to ensure that thebubble, as it rises, will properly enter the die to completely form thedesired article. In other words, the immersed end of the die guides thebubbles into thedie for proper formation. On the other hand, dependingon operating conditions and the nature of the material of the melt, itmay be desirable that the open end ofthe die not be immersed directly.in the melt, but be positioned in approximate contact with the meltsurface.

FIGURE 10 shows a modification of the die shown in FIGURE 1. In thisembodiment the die 20 comprises in addition a lower sleeve member 52immersed in the melt to the desired depth Y, the upper end of whichsleeve member 52 extends above the melt surface and tightly abuts ortelescopes without the open end of the 'die 20. The die 20 is sli-dably,but tightly, mounted in the lower sleeve member 52. The sleeve member 52may be in a fixed location at the desired depth of Y or preferably it ismounted for periodic immersion in the melt between blowing cycles. Inthis embodiment the upper member of the die 20 need only be brought intoa position wherein its lower open end fits within or abuts the lowersleeve member 52. With this arrangement the die 20 need not 20 to insurethe proper entry of the blown bubble of material into the die 20. Afterthe buble has been blown into the die 20, the upper die 20 can then bewithdrawn from the lower sleeve member 52 by a vertical upward movementso that the blown hollow article formed therein may be removed. Thesleeve member 52 being a separate member is periodically immersed in themelt between blowing cycles to remove any solidified metal that maycollect thereon. Thus, when the die 20 is remove-d from contact with thesleeve member 52 so that the blown article may be recovered, the lowersleeve member 52 is completely immersed in the melt and returned to itsoriginal position so that when the die 20 is returned to its abuttingposition with the sleeve member 52, the die 20 is ready for the nextblowing cycle.

The term die as used herein refers broadly to any means of forming theupwardly rising, expanding bubble of material and includes in particularthe die 20, as well as the die containing an abutting and retractable orimmersible sleeve member 52. It is recognized that the bubble ofmaterial may be captured and formed into the desired article by othermeans such as by the use of free forming techniques employing fluidstreams, such as air streams, or other forces to restrain and mold theliquefied metal bubble into the desired form prior to stabilizationthereof. The die serves as a limiting or restraining means to capturethe bubble.

It has been found that the blowing process of this invention can beapplied to metals other than aluminum. Examples of metals blown includebut are not limited to: Woods metal (M.P. 73 C.), lead (M.P. 327 C.),tin (M.P. 232 C.), cartridge brass (M.P. 1030 C.), zinc (M.P. 419 C.),and copper (M.P. 1083 C.). Small cylinders were successfully blown fromeach metal. Copper oxidized rapidly, but the rate was not so rapid thatan impervious crust formed on the surface of the bath. Right cylindersof copper were somewhat lacey in the thin sections at the rim of thecylinder due to oxidation.

In all cases, a 3" diameter by 6" deep alundum crucible was used as thecontainer. The metal was heated to melting by resistance wires aroundthe crucible. Normal fireback insulation surrounded the crucible.

The blowpipe tube was of Vycor glass tubing 9 mm. ID by 11 mm. OD. A gasdiffuser of 1" diameter containing 20-30 mil holes was affixed to theend of the tube with sodium silicate. The blow gas used was 97% nitrogenand 3% oxygen. The blowpipe was inserted beneath the molten metal sothat the top of the gas diffuser was 1% beneath the molten metalsurface. The molten metal was maintained at 60 F. above its meltingpoint. The total gas overpressure ranged from 100-250 mm. aboveatmospheric, depending on the metal. In the case of Woods metal and tinit was 100 mm. above atmospheric, with cartridge brass and copper it was250 mm. above atmospheric. Blow times were 0.4 second in all cases.

The mold was of gray cast iron. The ID of the die was 1%", wallthickness was A". The free length of the die was 3". The bottom of thedie was open, the top was fitted with a flat insert closure held byscrews so that there was a clearance of 0.010" between the periphery ofthe insert and inner wall of the die.

The blowing procedure was similar to that described with aluminum. Theopen end of the die was inserted into the liquid to a depth of 1", thevertical axis of the die having been aligned with the center of the gasdiffuser beforehand. Immediately after the die was inserted a gas bubblewas injected. Then the die was raised and the blown cylinder removed.

With the use of a protective nonoxidizing or inert atmosphere about abath of cast iron (M.P. 1230 C.), cast iron may be blown, while aprotective atmosphere would also reduce the oxidation of blown copper.

One of the distinguishing features of blown aluminum in contrast to castaluminum is the tendency toward crystal orientation in the blownaluminum. The degree to which this tendency exists depends on theparticular alloy and the particular blowing conditions. The reasons forthe preferential crystal growth are not fully understood; however,examples can be given to demonstrate the preferential growth.

A hypothesis of the microscopic mechanism of the blowing process may beset forth. At the instant a gas bubble is inserted beneath the moltenaluminum, nucleation starts because of the temperature differencebetween the gas and the melt or because of the formation of an unstablechemical intermediate which nucleates crystallization. At this stage,nuclei are probably not oriented due to agitation within the melt, atthe surface of the rising bubble. As the bubble rises into the die, themass of melt decreases and the bubble walls contact the relativelycooler die Walls. The bubble walls become pinned to the die walls andnuclei present initiate growth into the still semimolten mass of thebubble walls. The semimolten bubble walls are simultaneously stretchedand nucleated as the bubble rises to fill the die. The crystals growingfrom the nuclei propagate in the easiest growth direction under thestretching force. These steps take place rapidly in time because of theheat sink represented by the die walls. The maximum time within whichoriented crystal growth can occur is set by the temperature differencebetween the die walls and the semiliquid bubble walls and the difierencebetween the liquidus and solidus curves of the particular alloy. Thislatter statement is based on the simplifying assumption that the diewalls are essentially an isothermal heat sink during the instant ofsolidification. This is known not to be exactly true, because a thermalgradient is generated across the thin film air interface between thealuminum bubble walls and the inner die walls and between the die wallsand the bulk die material. The composition of the nuclei is dependent onthe temperature at which nucleation is initiated.

It can be seen that the extent to which preferred orientation of thegrowing crystals occurs is in some varying de gree dependent on thespecific alloy, the temperature of the molten metal bath, the speed ofinjection of th gas bubble, the gas temperature, the depth of theblowpipe beneath the surface, the die wall temperature and the thermalconductivity of the die material. The hypothesis postulates nucleatingsites formed within the semimolten mass of the aluminum bubble wallsimmediately before and during entrance into the die. These nuclei arepinned on entering the die and under the stretching force while in themold propagate crystal growth in the easy direction, into the bubblewalls. The process may be simply due to crystallites forming at theinterface between the cooler injected gas and the melt, or thenucleating sites may be due to contact with the cooler die walls.

Below are experimental data demonstrating preferred crystal growth intwo alloys of aluminum blown as taught in this patent application. Aside wall taken from a blown container or can was flattened, polishedand mounted in the usual technique for X-ray diffraction. The blownsamples were compared with a similarly flattened sample prepared from arapidly chilled, thick casting of the same molten metal. The X-ray tubeutilized had a copper metal target.

The four strongest diffraction lines were compared in' each case. Thepreferential orientation is evidently such that the 111 planes lieparallel to the sample plane.

One alloy examined was essentially a binary mixture of 8% silicon inaluminum. The other blown sample was made from commercial 2025 alloy.Two orientations (A.

and B) of the 8% silicon sample (at right angles to each other) wereexamined. One orientation of 2025 alloy was investigated. The relativeintensities of the four strongest lines are shown normalized to theintensity of reflection of the 111 plane at a value of 100.

In every case, the relative intensity of the 111' line in the blown canis stronger than the 111 line in a randomly oriented sample. The ratioof the intensity of the 111 line to the sum of the other lines supportsthis View. No estimate of the absolute amount of preferred orientationcan be made because of the lack of calibration against single crystalmaterial. However, tendency toward preferred orientation is significant.Inblown aluminum tested to date, the ratio of the principal diffractionline 111 to the sum of theother lines 200, 220 and 311 will be greaterthan 1.0.

The objects set forth above, among those made apparent from thepreceding description, are attained in the method of this inventionand'changes may be made in carrying out the method without departingfrom the scope ofthe invention." It is intended that all the mattercontained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense. 1

It is also to be understood that the language in the following claims isintended to cover all of the generic and specific features of theinvention herein described and all statements of the scope of theinvention which, as a matter of language, might be said to falltherebetween.

Having thus described our invention, we claim:

' 1. A method of manufacturing a hollow article from a bulk quantity ofliquid material which method comprises:

immersing the open end of a die a controlled depth into the liquidmaterial and thereby enclosing a portion of the exposed surface of theliquid material and leaving a die cavity above said enclosed surface;conducting a predetermined volume of a fluid under pressure to aposition in registry-with the open end of the die and at a controlleddepth beneath the surface of the liquid material; V f t T forming atsaid position a bubble of said fluid and causing said bubble to rise tosaid enclosed surface and to form a thin coherent film shell of saidbath material at andextending above said enclosed, surface, and causingthe bubble to expand so that its film wall continually expands and isstretched and is pressed into'contact with the internal surfaces of the'die cavity;

stabilizingthe bubble material in the die; and

I recovering the stabilized hollow article from thefldie.

2. The method as described in claim 1 wherein a liquid material is amolten metal alloy capable of forming an oxide film'when contacted inthe molten state by oxygen and wherein the fluid 'under pressure is agas containing oxygen, in a controlled proportion;

3." The method as described in claim 1 wherein the material isstabilized by controlling the temperature of the die into which thebubble is blown.

4. The method as described in claim 1 whereinthe fluid under pressure isa gas, the liquid material is a molten metal, and the bubble is formedby introducing thepr'edetermined volume of the gas into the one end of ablow pipe wherein the other end of the blow pipe has an orifice disposedin registry with the open endof the die" and at a controlled depthmaterial.

5. A method as described in claim 1 wherein the fluid under pressure isa gas containing a controlled amount of oxygen and the liquid materialis a molten aluminum alloy and wherein the aluminum is stabilized bysolidification of the bubble in the die.

6. A method ofcontinuously manufacturing a plurality of hollow articlesfrom a bulk quantity of a liquid material and an open end diecharacterized by an internal die cavity and including a detachable openend lower sleeve member which method includes:

immersing the open end of the sleeve member of the die a controlleddepth into the liquid material and thereby enclosing a portion of theexposed surface of the liquid material and leaving a die cavity abovesaid enclosed surface;

conducting a predetermined volume of a gas under pressure, to a positionin registry with the open end of the sleevemember and at a controlleddepth beneath the surface of the liquid material;

beneath the surface of the liquid formin gand introducing a bubble ofsaid material into the immersed open end of the sleeve member and die,said bubble being expanded by said gas under pressure. so that its filmwall continually expands and is stretched and is pressed into contactwith the internal surfaces of the die cavity; stabilizing the bubblematerial in the die cavity; separating the die from the sleeve member;

' recovering the hollow article formed from the separated die;

repositioning the die in mating contact with sleeve memblowing a bubbleof the molten metal expanding above the surface of the bath from acontrolled depth beneath the surface of the molten metal employing apredetermined volumeof gas under pressure to form the bubble;

forming the molten bubble so blown into a hollow article of the desiredshape; and

solidifying the. formed bubble.

9. A method as described in claim 8 wherein the molten metal is selectedfrom a group of metals consisting of aluminum, tin, lead, zinc, copperand alloys thereof.

'10. A method as described in claim 8 wherein the molten metal iscapable of forming a thin oxidized surface and the gasunder pressureemployed to blow the bubble is a gas with a controlled proportion ofoxygen.

11. A method as described in claim 10 wherein the molten bubble isformed into a desired hollow metal article by introducing the bubbleinto the open end of a die immersed a predetermined distance into themolten metal which die is maintained at a temperature less than that ofthe molten metal.

12. A method of fabricating a hollow article which method comprises:

heating a quantity of molten material to a controlled temperature toprovide a bath of fused material;

immersing the open end of a die a controlled depth into the said bathand thereby enclosing a portion of the exposed surface of the liquidmaterial and leaving a die cavity above said enclosed surface, said diehaving an inner surface which defines the outer shape of the hollowarticle;

providing a blow pipe adapted to be connected at its one end with asource of fluid under pressure and having an orifice at its other enddisposed in registry with the open end of the die and at a controlleddepth beneath the surface of the bath;

blowing a bubble of said material into the open end of the die and intointimate contact with the inner surface of the die by connecting the oneend of the blow pipe with a source of fluid under pressure to form atsaid orifice a bubble of said material and cause it to rise to saidenclosed surface and to form a thin coherent film shell of said bathmaterial at and extending above said enclosed surface, and thereby causethe bubble to expand so that its film wall continually expands and isstretched and is pressed into contact with the internal surfaces of thedie cavity;

solidifying the blown article;

removing the open end of the die from the bath; and

recovering the article formed.

13. The method of claim 12 which includes controlling the temperature ofthe material in the bath to vary the wall thickness of the articleformed.

14. The method of claim 12 which includes controlling the temperature ofthe die to stabilize the bubble blown into the die.

15. The method of claim 12 which includes controlling the overpressuredeveloped within the die to permit the bubble to contact the internalwalls of the die prior to solidification.

16. The method of claim 12 wherein said fluid under pressure used toblow the bubble is a gas containing a controlled proportion of oxygenand said material is a metal alloy capable of forming an oxide layerwhen formed with the blowing gas.

17. The method of claim 12 which includes controlling the pressure andamount of the fluid used to blow the bubble to vary the size of bubble.

18. The method of claim 12 which includes developing :an under pressureless than the blowing pressure within the interior of the die when thebubble is introduced into the die.

19. The method of claim 12 which includes controlling the orifice of theblow pipe to vary the diameter of the bubble blown.

20. The method of claim 12 which includes control- Hing the depth of theorifice of the blow pipe below the :surface of the bath of the materialto vary the wall thick- :ness of the article.

21. The method of claim 12 wherein said material is :an aluminum alloyand the fluid under pressure is a gas containing about 1 to 3 parts perhundred of oxygen.

22. The method of claim 12 which includes controlling the depth of theopen end of the die to vary the overpressure within the die.

23. A method of fabricating a hollow sealed article which comprises:

forming a hollow article in accordance with the method of claim 12 andprior to removing the open end of the die from the bath placing animperforate plate member in sealing engagement with the open end of thmolt. I9 pr d .2 sealed lid of controlled thickness;

removing the sealed die from contact with the bath;

and recovering from the die a hollow sealed article. 24. A method offabricating a hollow metal article which method comprises:

melting a quantity of a metal to a controlled temperature to form a bathof molten metal alloy;

immersing the open end of a die to a controlled depth in the bath andthereby enclosing a portion of the exposed surface of the molten metaland leaving a die cavity above said enclosed surface; disposing a blowpipe having at its one end an orifice disposed at a controlled depthbeneath the surface of the bath; and in registry with the open end ofthe die;

blowing a bubble of said metal alloy above the surface of the bathand-into the open end of the die by introducing a predetermined volumeof gas under pressure into the other end of said blow pipe to form thebubble and cause it to rise to said enclosed surface and to form a thincoherent film shell of said metal at and extending above said enclosedsurface, and causing the bubble to expand so that its film wallcontinually expands and is stretched and is pressed into contact withthe internal surfaces of the die cavity; and solidifying the blownarticle formed within the die.

25. The method of claim 24 wherein said gas under pressure comprises agas capable of setting free a controlled amount of oxygen when incontact with the molten alloy.

26. The method of claim 24 wherein the metal is an aluminum alloy andthe gas contains from about 1 to 3 parts per hundred of oxygen.

27. A method of fabricating a hollow aluminum article comprising:

melting a quantity of an aluminum alloy;

immersing a die having an open end a predetermined distance into themolten aluminum alloy and thereby enclosing a portion of the exposedsurface of the liquid alloy and leaving a die cavity above said enclosedsurface;

disposing the orifice of a blow pipe in a registry with the open end ofthe die and a predetermined distance below the surface of the moltenaluminum alloy;

and I blowing a bubble of the molten aluminum alloy into the die usingto blow the bubble an atmosphere containing free oxygen passed throughthe blow pipe to cause the bubble of alloy to rise to said enclosedsurface and to form a thin coherent film shell of said bath material atand extending above said enclosed surface, and causing the bubble toexpand so that its film wall continually expands and is stretched and ispressed into contact with the internal surfaces of the die cavity.

28. The method of claim 27 wherein the atmosphere used to blow thebubble comprises oxygen and an inert gas.

29. A method of fabricating a hollow metal article from a fused bath ofa metal'which method comprises:

suspending the open end of a die slightly above the surface of themolten metal and in a position to cap ture within the die a bubble ofmolten metal from the surface of the metal; 7 providing a blow pipeadapted to be connected at its one end with a source of gas underpressure and having an orifice at its other end disposed in registrywiththe open end of the die and at a controlled depth beneath the surface ofthe bath;

blowing a hollow bubble of the molten metal containing gas into the openend of the die by connecting the one end of the blow pipe with a sourceof gas under pressure;

solidifying the blown article in the die; and

recovering the article formed.

30. The method of claim 29 wherein the gas contains a controlledproportion of oxygen.

31. The method of claim 29 which includes inducing at least a partialvacuum within the cavity of the die to aid in capturing the bubblewithin the suspended die.

32. The method of manufacturing a hollow metal article from a bulkquantity of molten metal which method comprises:

suspending the open end of a die above the surface of the molten metal;

forming a hollow gas-containing bubble of the metal and introducing saidbubble into the suspended open end of the die and into contact with theinternal surfaces of the die by introducing a predetermined volume ofgas under pressure into the metal through a blow pipe havig an orificedisposed in registry with the open end of the die and at a controlleddepth below the surface of the molten metal;

stabilizing the bubble material in the die; and

recovering the stabilized hollow metal article from the die.

33. A method of manufacturing a hollow aluminum article from a bulkquantity of molten aluminum alloy which method comprises:

suspending the open end of a die above the surface of the molten alloy;

conducting a predetermined volume of gas under pressure and containing acontrolled amount of oxygen to a position in registry with the open endof the die and at a controlled depth beneath the surface of the moltenalloy;

forming and introducing a hollow bubble of the molten alloy andcontaining said gas into the suspended open end of the die and intocontact with the internal surfaces of the die; solidifying the bubble inthe die; and recovering the solidified aluminum article from the die.

References Cited UNITED STATES PATENTS 12,086 12/1854 Tiebe et al.22-200 26,913 1/1860 Leonard 29-183 272,044 2/ 1883 Harker 22-200397,641 2/ 18.89 Halford et a1. 22-202 740,874 10/1903 Krause 22-202788,142 4/1905 Pease -192 788,144 4/1905 Pease 65-192 852,396 4/1907Pease 65-192 X 901,361 10/1908 McCarty 22-209 1,435,292 11/1922 Grey22-209 1,699,592 1/1929 Kadow.

1,725,144 8/1929 Kadow 22-209 X 2,002,875 5/1935 Woods 65-88 2,005,1756/1935 Adams 29-183 2,019,046 10/ 1935 Delpech 49-31 2,174,930 10/ 1939Soubier 65-263 X 2,209,877 7/ 1940 Ferngren 18-58 2,751,289 6/1956Elliott -20 3,013,311 12/1961 Meissner 18-57 3,184,296 5/ 1965 Schaich65-73 FOREIGN PATENTS 803,122 3/ 1951 Germany. 193,945 2/ 1923 GreatBritain.

I. SPENCER OVERHOLSER, Primary Examiner. V. K. RISING, AssistantExaminer.

8. A METHOD OF FABRICATING A HOLLOW METAL ARTICLE FROM A LIQUID BATH OFA MOLTEN METAL WHICH METHOD COMPRISES: BLOWING A BUBBLE OF THE MOLTENMETAL EXPANDING ABOVE THE SURFACE OF THE BATH FROM A CONTROLLED DEPTHBENEATH THE SURFACE OF THE MOLTEN METAL EMPLOYING A PREDETERMINED VOLUMEOF GAS UNDER PRESSURE TO FORM THE BUBBLE; FORMING THE MOLTEN BUBBLE SOBLOWN INTO A HOLLOW ARTICLE OF THE DESIRED SHAPE; AND SOLIDIFYING THEFORMED BUBBLE.